Gradient elution in HPLC: How does the solvent composition behave during a gradient program?

Introduction / Context:
Gradient elution is a powerful tool to resolve mixtures with a wide range of polarities. By changing the mobile-phase composition over time, one can elute early the weakly retained species and still bring out strongly retained species within the same run.

Given Data / Assumptions:

• Mobile phase is typically a binary mix (e.g., water–acetonitrile or water–methanol).
• Stronger solvent fraction is increased with time to decrease retention.
• Gradients can be linear (continuous) or stepwise.

Concept / Approach:
Retention factor decreases as the proportion of stronger solvent increases. A programmed change in composition improves peak shape, reduces analysis time, and helps elute late-retaining components. Isocratic elution, in contrast, uses constant composition.

Step-by-Step Solution:
Define gradient elution: time-dependent solvent composition.Recognize acceptable profiles: linear ramps or discrete steps.Select the option stating the composition changes continuously or stepwise.

Verification / Alternative check:
HPLC system methods show programmed gradients (%B vs time tables), confirming variable composition during the run.

Why Other Options Are Wrong:

• Constant composition defines isocratic, not gradient.
• “Both” cannot be true since constant and changing are mutually exclusive.
• “None” is incorrect because (b) is correct.

Common Pitfalls:
Ignoring dwell volume and gradient delay when transferring methods between instruments; these impact the effective gradient reaching the column.

Final Answer:
is changed continuously or in a series of steps